Superconducting cable termination

ABSTRACT

Disclosed is a termination that connects high temperature superconducting (HTS) cable immersed in pressurized liquid nitrogen to high voltage and neutral (shield) external bushings at ambient temperature and pressure. The termination consists of a splice between the HTS power (inner) and shield (outer) conductors and concentric copper pipes which are the conductors in the termination. There is also a transition from the dielectric tape insulator used in the HTS cable to the insulators used between and around the copper pipe conductors in the termination. At the warm end of the termination the copper pipes are connected via copper braided straps to the conventional warm external bushings which have low thermal stresses. This termination allows for a natural temperature gradient in the copper pipe conductors inside the termination which enables the controlled flashing of the pressurized liquid coolant (nitrogen) to the gaseous state. Thus the entire termination is near the coolant supply pressure and the high voltage and shield cold bushings, a highly stressed component used in most HTS cables, are eliminated. A sliding seal allows for cable contraction as it is cooled from room temperature to ˜72-82 K. Seals, static vacuum, and multi-layer superinsulation minimize radial heat leak to the environment.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of U.S.Provisional Application No. 60/329,234, filed Oct. 12, 2001.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a termination for connectingsuperconducting and high temperature superconducting (HTS) cablesoperating at sub-ambient temperatures to cables operating at ambienttemperature.

[0003] It is known that superconductors are metals, alloys, or oxidesthereof, and in general are compounds having practically zeroresistivity below a transition temperature, i.e. the criticaltemperature. A superconducting cable must be operated below its criticaltemperature, therefore it is cooled during use by, for example,cryogenic cooling fluids. Metal and alloy superconductors have criticaltemperatures below 20° K. while metal oxide (ceramic) superconductorshave higher critical temperatures on the order of 80° K. thusdistinguishing them from the former materials and separating them into aclass known as high temperature superconductors that are used to makeHTS cables. Because of the brittleness of high temperaturesuperconductors, the cable making material is presently manufactured inthe form of tapes known as HTS tapes.

[0004] Because of their negligible resistance, superconducting powercables lose only about one-half percent of power during transmission,compared to a 5 to 8 percent loss of traditional power cables anddeliver about three to five times more power through the same area thantraditional power cables. As the rapid growth of urban areas increasesdemand for electricity, the ability of HTS cables to transmit more powerwhile using equivalent amounts of space as traditional cables areincreasingly important.

[0005] To be useful, a superconducting cable must have terminations suchthat the cold superconductor may be connected to a conventionalresistive conductor in an ambient temperature environment. The twoprimary functions carried out by a superconducting cable termination areproviding transition from the cryogenic superconducting environment toambient conditions and transitioning the large radial voltage gradientin the cable to the much lower gradient tolerable after termination.

[0006] Generally, an HTS cable has a coaxial configuration comprised ofan energized inner superconductor (phase or line), at least one layer ofelectrical insulating material, and an outer layer of superconductorplaced at zero potential (neutral, ground, or shield). Multiple layersof energized superconductor and electrical insulation may be present insome cables to transmit three phase power. An HTS cable is generallymade by winding HTS tapes over a hollow tube known as a former. Theformer provides mechanical support for the HTS tapes and electricalinsulation as well as a path for cryogenic fluid circulation from oneend of the cable to the other for cable cooling. The coolant, in someHTS cable designs, permeates the cable structure and thereby becomes animportant part of the electrical insulation. In this function, thecoolant must also be kept at a pressure where bubbles do not form duringoperation and the coolant pressure may then be above ambient pressure.HTS cable is housed in a conduit with thermal insulation to keep thecable at the desired temperature and having sufficient strength toaccommodate the pressure of the cooling fluid and protect the cable fromharm. The conduit also provides an additional path for cryogenic fluidcirculation from one end of the cable to the other for cable cooling.Terminations are located on each end of the HTS cable to affect thetransition from the superconducting cable, generally cooled bypressurized cryogenic fluid such as liquid nitrogen, to externalbushings at ambient temperature.

[0007] Various types of terminations have been used in the prior art,but these terminations are complex, subject to stress and susceptible tofailure.

[0008] A common prior art design has two sets of bushings, a coldbushing and a warm bushing, at two separate boundaries. At the firstboundary the cold bushing separates the HTS cables cooled by cold,pressurized liquid nitrogen from another region that is warmer andeither is in a vacuum or is filled with an insulating gas such asnitrogen or SF6. At the second boundary the warm bushing separates thevacuum or insulating gas region from ambient conditions (i.e. 295° K.and one atmosphere). The cold bushing in such designs is a highlystressed component and prone to failure. The bushing experiencessignificant thermal/mechanical stresses during cooldown of the cable andmust be designed for cable current (several kA) and, for the innerconductor, has to have sufficient solid insulation for the rated voltage(˜10-100 kV). The bushing must also have sufficient electricalinsulation to withstand the rated voltage.

[0009] In one known embodiment, described by C. Bogner in “Transmissionof Electrical Energy by Superconducting Cables”, pages 5145-16 in S.Foner and B. B. Schwartz ed., Superconducting Machines and Devices, NATOAdvanced Study Institute, Entreves, Italy, 1973, Plenum Press (1974) aterminal for a single-phase superconducting cable comprises a vacuumcontainer inside which a casing filled with low-temperature liquidhelium is disposed.

[0010] U.S. Pat. No. 6,049,036 discloses a terminal for connecting amultiphase superconducting cable to room temperature electricalequipment. The terminal includes a casing with cooling fluid, insidewhich three cable superconductors are connected with a resistiveconductor the end of which is connected to the room temperatureequipment phases at the outside of the casing. The design featuresinternally cross connections between the three shield conductors at thecold end eliminating the need for the shield conductors to ambientconditions, although an external connection is provided to establishground potential. In this design, the internal portion of the resistiveconductor ends are filled by gaseous coolant that forms an interfacewith the liquid coolant somewhere along the resistive conductor and thisinterface is held in place by gravity, thus vertical orientation isrequired in this invention. Further, this invention has a high voltageinsulator that forms a vacuum boundary that extends from roomtemperature to coolant temperature.

[0011] U.S. Pat. No. 4,485,266 discloses a termination for connecting asingle coaxial superconducting power transmission line to an ambientbushing that operates in the horizontal position. The invention has acompletely sealed horizontal conduit that connects the coldsuperconducting cable to a room temperature sulfurhexafluoride insulatedbushing. The sealed conduit is a very complex structure that provideselectrical insulation between phase and shield as they warm andtransition to normal conductors, each of which has its own independentlycooled heat exchanger that controls the temperature gradient along theconductor.

[0012] U.S. Pat. No. 3,902,000 discloses a termination for connecting asingle coaxial superconducting cable to an ambient temperature bushing.The patent provides for a low temperature stress cone to expand thedimensions of the insulation prior to encountering the verticaltemperature gradient region. This is done because the coolant, helium,has poor dielectric properties in the warm gaseous state. Gaseouscoolant is vented from the top of the termination to provide cooling forthe temperature transition zone. The inner conductor is connected to aconventional bushing having conventional dielectric fluid at the warmend.

[0013] Prior art terminations utilized either vertical configuration ora very complicated horizontal section with independent cooling circuitsto control temperature gradients in the transition zone between thesuperconducting and normal conducting cables. The present inventionconsiderably simplifies the design of terminations for HTS cables byusing a unique and innovative technique employing the thermal gradientalong the termination's copper conductors to eliminate the requirementfor vertical orientation or independent cooling circuits. This producesan HTS cable that is more reliable due to the inherent simplicity of thetermination design.

BRIEF SUMMARY OF THE INVENTION

[0014] Superconducting cables consist of one or more electricallyinsulated superconducting conductors contained in a hermetically sealedthermally insulated conduit. Said superconducting cable being maintainedat a temperature below the superconducting transition temperature byflowing a coolant such as liquid nitrogen through the conduit. Each endof the superconducting cable conduit is connected to a termination thatprovides a means for connecting external, ambient temperature, normalconductor connectors to the superconducting conductors. Each of the twoterminations consists of a set of electrically insulated normalconductors having one end maintained at a temperature below thesuperconducting transition temperature that is electrically connected toits corresponding superconducting cable and having the other endconnected to the internal connector of an ambient temperature bushing.The normal conductors thus have a large temperature difference from oneend to the other.

[0015] The termination consists of the normal electrically insulatedconductors contained in a thermally insulated conduit. The terminationconduit consists of three distinct regions. A cold end housing formaking connections between the normal conductors and thesuperconductors. An ambient temperature housing for making connectionsbetween the normal conductors and the internal connection of the ambienttemperature hermetic bushing. A transition duct connects the coldhousing and the ambient housing through which the normal conductors andinsulators pass. The transition duct is sized so that the insulators andconductors completely fill the duct. Sealant compounds, elastomer seals,and mechanical seals close the gaps between the insulators, conductors,and transition duct. One or more capillary passages through, or parallelto, the termination duct connects the cold housing end to the ambienthousing end to maintain pressure equilibrium across the terminationduct, thereby limiting liquid coolant from flowing from the cold housingto the warm housing. The conductors and transition duct are of a sizeand length so as to minimize the heat flow through the normal conductorsfrom the ambient temperature housing to the cold housing.

[0016] The present invention is an innovative termination that connectsthe high temperature superconducting (HTS) cable regions which areimmersed in pressurized cryogenic fluid such as liquid nitrogen to thehigh voltage and neutral (shield) external bushings at ambienttemperature and pressure. The termination consists of a splice betweenthe HTS power (inner) and shield (outer) conductors and concentricresistive conductors (copper pipes) in the termination.

[0017] There is also a transition from the dielectric tape insulatorused in the HTS cable to G-10 insulators used between and around thecopper pipe conductors in the termination.

[0018] The invention consists of a feed and a return end or terminationsdesignated by the flow of cryogenic fluid. Each termination has a warmand cold end. At the warm end of the termination the copper pipes areconnected via copper braided straps to conventional warm externalbushings which have low thermal stresses.

[0019] Thus, the termination allows for a natural temperature gradientin the copper pipe conductors inside the termination which enables thecontrolled flashing of the coolant, i.e. pressurized liquid nitrogen togaseous nitrogen. Thus the entire termination is near the nitrogensupply pressure thereby eliminating the high voltage and shield coldbushings, a highly stressed component used in prior art HTS cables.

[0020] The copper conductors transfer heat absorbed from the outside atambient temperature and heat produced by current passage under aresistive effect, to the cryogenic liquid coolant which passes throughthe resistive conductors, which heats up and flashes to gas.

[0021] Other aspects of the design include: (1) a sliding seal to allowfor cable contraction as it is cooled from room temperature to ˜72-82 Kand (2) specialized seals and static vacuum with multi-layersuperinsulation to minimize radial heat leak to the environment.

[0022] The present inventive termination can be used by cablemanufacturers and the electric utility industry in replacing theoverburdened infrastructure of conventional copper cables havingoil/paper insulation with a new generation of more efficient HTS cables,especially in urban areas where the higher current density of the HTSconductors would allow increased capacity in existing underground cabletunnels.

[0023] One object of the present invention is to provide a simplifiedHTS cable termination.

[0024] It is also an object of this invention to provide an HTS cabletermination that does not require a cold bushing.

[0025] A further object of this invention is to provide a terminationfor connecting an HTS conductor and shield to copper conductors forelectrical power transmission.

[0026] Another object of this invention is to provide for a terminationwhich is near the supply pressure of the cryogenic coolant.

[0027] It is a further object of the invention to provide a terminationpartially formed of pressurized piping made of fiberglass sections.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a schematic representation of one embodiment of thetermination of the present invention.

[0029]FIG. 2 is a cross sectional view showing an HTS cable withterminations of the present invention.

[0030]FIG. 3 is a cross sectional view showing one embodiment of thesuperconducting cable feed termination.

[0031]FIG. 4 is a cross sectional view showing an embodiment of thesuperconducting cable return termination.

[0032]FIG. 5(a) is a cross sectional view showing one embodiment of ajoint between the superconducting cable conduit and the terminationconduit.

[0033]FIG. 5(b) is a cross sectional view showing an alternativeembodiment of a joint between the superconducting cable conduit and thetermination conduit.

[0034]FIG. 6 is a cross sectional view showing an HTS cable totermination splice in detail.

[0035]FIG. 7 is a detail of the cold end sliding seal between thecoolant jacket and the outer normal conducting pipe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] While the exact design of the superconducting cable may vary, theembodiment of this invention described and illustrated here is forconnecting a coaxial superconducting cable consisting of asuperconducting center phase conductor and an outer superconductingshield conductor, to a pair of copper conductors that make utilizationof the cable for electrical power transmission possible. The presentembodiment is designed for continuous 1.25 kA operation at 7.2 kVACoperation and 110 kV BIL and has been operated continuously at 13 kVACand withstood 120 kV impulse. The same principles can be used to designsuperconducting terminations and splices that have multiple phaseconductors operate at different current and voltage levels.

[0037]FIG. 1 shows a schematic representation of the superconductingcable termination of the present invention. Referring to FIG. 1,superconducting cable 101 is shown in the conduit which is surrounded bytermination conduit 109. Cold section 403 in termination conduit 109 isfilled with coolant 413 and contains splice 412. Thermal insulation 411surrounds superconducting cable 101 and conduit 109. Transition duct 404is adjacent cold section 403 and positioned between cold section 403 andambient temperature section 405. Ambient temperature section 405contains gaseous coolant 406 at conduit pressure and ambienttemperature. Internal connections 407 and external connections 408 arelocated in ambient section 405. Hermetic bushings 409 are intermediatebetween external connections 408 and ambient temperature section 405. Apressure equalization capillary 410 connects ambient temperature section405 and cold section 403. A detail of the internal connectionsattachment to the splice section end present in ambient temperaturesection 405 shows thermal insulation 414 adjacent electrical insulation122 which is adjacent copper pipe conductor 114. Electrical insulation123 is situated between copper pipe conductor 114 and copper pipeconductor 115. Electrical stress relief material 29 abuts electricalinsulation 123 and copper pipe conductor 114. Internal connector 118 isadjacent copper pipe 114 and one of the external connections 408. Thesuperconducting cable is contained in a hermetically sealed, thermallyinsulated cable conduit. Each end of the superconducting cable conduitis connected to a termination that provides a means for connectingexternal, ambient temperature, normal conductors to the superconductingconductors. Each of the two terminations are identical except forconnections made to accommodate coolant flow. The superconducting cableis maintained at a temperature below the superconducting transitiontemperature by flowing a coolant, such as liquid nitrogen, through theconduit. The coolant also cools the splice between the superconductorand normal conductor in the termination. The termination consists of aset of electrically insulated normal conductors, shown as copper pipes114 and 115 in FIG. 1, having one end maintained at a temperature belowthe superconducting transition temperature that is electricallyconnected to its corresponding superconducting conductor, in the regionthe splice 412, and having the other end connected to the internalconnector of an ambient temperature bushing. The normal conductors,copper pipes, are contained in a transition duct 404 that has a largetemperature difference from one end to the other. Therefore, thetermination conduit 109 consist of three distinct regions: a coldsection 403 for making connections between the normal conductors and thesuperconductors, an ambient temperature section 405 for makingconnections between the normal conductors and the internal connection ofthe ambient temperature hermetic bushing, and a transition duct 404connecting the cold section and the ambient section through which thenormal conductors and insulators pass. The transition duct is sized sothat the insulators and conductors completely fill the duct. Sealantcompounds, elastomer seals, and mechanical seals close the gaps betweenthe insulators, conductors, and transition duct. The cold section andtransition duct share a common thermally insulated conduit. One or morecapillary passages through, or parallel to, the termination ductconnecting the cold housing end to the ambient housing end maintainpressure equilibrium across said termination duct, thereby limiting theliquid coolant flow from the cold housing to the warm housing. FIG. 1illustrates the pressure equalization capillary 410 external andparallel to the transition duct The conductors and transition duct areof a size and length so as to minimize the heat flow through the normalconductors from the ambient temperature section to the cold section ofthe termination conduit.

[0038]FIG. 2 shows a simplified representation of an HTS cable with twoterminations. HTS cable 11 is housed within cable conduit 12 which isprovided with a vacuum jacket that limits radial heat transfer to thecable from the surroundings. HTS cable 11 is normally a multilayerstructure wound in a coaxial configuration around a former that ishollow in the center to allow a flow of cryogenic cooling fluid, such asliquid nitrogen. The former may be made of flexible materials, includingpolymers and metals. Advantageously the former is a stainless steel hosewith perforations to allow the cryogenic fluid to surround and permeatethe HTS cable. Thermally insulated conduit 12 maintains the coolingfluid at desired operating temperature by retarding heat flow to thecoolant, maintains the pressure of the cooling fluid, and protects thecable. The present invention consists of a feed end termination 13 and areturn end termination 14. The ends are called terminations in the usualsense of a coaxial cable termination in that they allow the centerconductor to be accessed while preventing breakdown to the shield, butthey also have the additional functions of accessing coolant to the HTScable, and interfacing the copper conductor to the HTS conductor.

[0039] The designation of feed termination 13 and return termination 14in FIG. 2 is used in this specific embodiment to refer to the fact thatthe coolant is circulated in a counter-current manner entering at flowpipe 24 and exiting a flow pipe 25. Hence the designation feed for thetermination that interfaces with the external coolant circulation systemin the counter-current cooling configuration and the designation returnfor the termination that internally reverses the coolant flow directionin the counter-current flow configuration. In the counter-current flowconfiguration, coolant is supplied to pipe 24 and flows through the HTSto normal conductor splice shown in FIG. 1, through the center of theHTS cable from feed termination 13 to return termination 14, where itpasses through the HTS to the normal conductor splice to the outside ofHTS cable 11, returning to the feed termination 13 through the annularspace between HTS cable 11 and the cable conduit 12, then exiting thesystem through flow pipe 25. The coolant can also be routed in aco-current configuration through the system by introducing it into bothflow pipes 24 and 25 and removing it at flow pipe 26 in the returntermination. Pipes 24, 25, and 26 may also function as ports forpressure relief, for instumentation, and for attachment of externalpressure equalization capillary for the ambient temperature part of thetermination 15. Port 27 is provided to allow external pressureequalization across the joint between HTS conduit 12 and terminationconduits 13 and 14. External electrical connections are made in theterminations through hermetic bushings to the phase at 16 and the groundat 20. Within the terminations, the splice between the HTS and normalconductor is made in the cold section of the termination at 22 for thephase and 23 for the shield. Normal conducting copper pipes 21 pass fromthe cold section to the ambient temperature section of the terminationthrough a duct with sliding seals that allows conductor motion. The ductis either hermetically sealed, if an external pressurization capillaryis used, or has an internal flow capillary if internal pressurization isemployed. Internal connections between the copper pipe conductors 21 andthe hermetic bushings 16 and 20 are made at ambient temperature usinginternal clamps 18 and copper braid straps 17. Flexible copper braidstraps 17 allow the cable to move relative to said hermetic bushings 16and 20 without transmitting mechanical stress to the bushings.Conversely, flexible copper braid straps 17 allow unconstrainedcontraction and expansion of the cable with temperature. Electricalstress relief material 19 is applied at the end of the shield copperpipe at ground potential to prevent electrical breakdown across theelectrical insulation to the coaxial pipe at phase potential.

[0040]FIGS. 3 and 4 show detailed cross sections of the feed and returnterminations, respectively. These terminations are identical except forthe coolant pipes 24, 25, and 26, the absence of seal 127 in the returntermination (FIG. 4), and a difference in outer sleeve 107, details thatwill be discussed later.

[0041] Advantageously the present invention uses a coolant liquidnitrogen, at pressures in excess of atmospheric pressure but less than150 pound per square inch (psi), therefore 150 psi-class components areused at all pressure boundaries. Referring to FIG. 3, HTS cable 102 ishoused in a vacuum insulated cable conduit 132 and interfaces with thetermination conduit at flange 101 forming a warm pressure boundary. Theannular gap between cable 102 and termination conduit 109,advantageously about 0.1 inch in this embodiment, may be packed withGore-Tex packing 240 and grease-impregnated fiberglass sleeving 241 asindicated in FIG. 5a. Alternatively, the annular gap may be packed withGore-Tex packing 240, dry fiberglass sleeving 242, and fiberglass filledgrease 243 as indicated in FIG. 5b. Said grease should have propertiessimilar to high temperature silicone grease having a wide temperaturerange and high dielectric strength. Said fiberglass filled grease isformulated by adding 33% by weight {fraction (1/32)}″ long glass fibersto said grease. Said grease packings form a hermetic seal that preventscoolant from migrating axially down the annulus. Flange 27 may beattached by a capillary to the coolant return line to equalize thepressure across the packing. In an alternate embodiment of the presentinvention, the termination may be designed to connect vertically upwardfrom the cable, in which case the grease impregnated fiberglass sealedbayonet may be replaced by a standard liquid nitrogen bayonet fitting.Termination conduit 109 in this embodiment also utilizes vacuum thermalinsulation. When vacuum insulation is employed the insulating quality ofsaid conduit may be enhanced by placing layers of superinsulation in thevacuum space to reduce radiation heat transfer and getter and/oradsorbent material may be attached to the cold surface internal tovacuum space 235 to help maintain vacuum over long periods of time.Conduit 109 is also equipped with combination pump-out port with apressure relief plug 236 and a vacuum gauge tube 237, as is commonlydone with vacuum insulated cryogenic equipment. Fittings 110, 113, and131 along with the hermetic bushing and other flanges constitute thehousing for the ambient temperature portion of the termination (i.e. 15in FIG. 2). Advantageously the present invention uses fiberglass andepoxy composite fittings for this purpose. Other materials may be usedthat provide either adequate standoff or insulation to ground to preventelectrical breakdown to internal high voltage components across thegaseous coolant that provides dielectric strength in this section of thetermination. Design of the ambient temperature housing can takeadvantage of the fact that pressurized liquid nitrogen has a highdielectric breakdown strength. An optional relief valve 121 may beprovided to prevent overpressure should liquid coolant enter andsuddenly evaporate in this section of the termination. The pressure ofthe relief valve is selected to give some operating margin above normalsystem operating pressure and should be sized for maximum boil-off rateduring accident conditions. The ambient temperature region is designedto be at ambient temperature when the cable is in service carrying fullcurrent. When no current, or reduced current, is applied and coolantflow is continued, the ambient temperature section will cool belowambient temperature. To accommodate this additional cooling, gaskets andother components are selected for service at the reduced temperature.Alternatively, heat may be applied to the system using thermostaticallycontrolled heat blankets or tapes to maintain ambient temperature. Spoolpiece 110 and other housing components in this region may be made ofaluminum, or other high thermal conductivity material, to facilitateheat transfer from surroundings or applied heating elements. Heattransfer to surroundings may also be enhanced by addition of externaland internal cooling fins or through the use of heat pipes. These arethe basic components of the pressure boundary of said termination. Theexact size of these various components is determined such that they canadequately house the termination internals with sufficient clearances toprevent electrical breakdown and interface with the external system.

[0042] The sizing of the termination internals beginning with thetransition section of the termination is as follows. The conductors andtransition duct are of size and length so as to minimize the heat flowthrough the normal conductors from the ambient temperature housing tothe cold housing. The present invention is designed for continuous 1.25kA operation at 7.2 kVAC operation and 110 kV BIL. Two concentric,electrically insulated, copper pipes carry current through thetransition section to the HTS cable phase and shield conductor, FIGS. 3,112 and 114 respectively. The pipes are sized according to theoptimization principles outlined by R. McFee in “Optimum Input Leads forCryogenic Apparatus” pages 98-102 in The Review of ScientificInstruments Volume 30 (1959), but constrained by available pipe sizesand the annular separation required for adequate electrical insulation.Advantageously phase pipe 112 is 1.25 inch ASTM-B-188 standard wallcopper pipe (1.25 inch I.D. by 1.66 inch O.D.) and has an optimal lengthat full current with a temperature gradient from 300° K. to 77° K. of 54inches and shield pipe 114 is ASTM F68 2.5 inch O.D. by 0.065 inch wallcopper tubing and has an optimal length at full current with atemperature gradient from 300° K. to 77° K. of 42.5 inches. Optimallengths are the nominal distances from the copper pipe's respectiveinternal connector to the point that the copper pipe first encountersits respective liquid nitrogen coolant. In the ambient temperaturesection additional phase pipe length extends under the internal phaseconnector 218 and additional shield pipe length extends under theinternal shield connector 118, plus an additional inch to allowapplication of the stress control material 29. Internal connectors 118and 218 are preferably clamshell copper connectors that are bolted topipes 114 and 112. Internal connectors 118 and 218 advantageously arebrazed to flexible copper braids 117 and 217 and said braids haveappropriate attachments for interfacing with bushings 20 and 16.Internal phase connector 218 serves the additional function of securingthe position of tube 123 and 111, advantageously using set screws. Inthe cold section additional pipe length extends into the splice toprovide adequate surface area for heat transfer for the heat load atfull current plus additional length required for mechanical attachmentsto the superconducting cable. Further additional pipe length may beadded to accommodate expansion and contraction of the cable by havingmore length to move back and forth in the duct. The annular spacebetween the pipes is occupied by tube 123 fabricated of filament woundglass impregnated with epoxy having properties similar to G10. G10 hasthermal expansion properties similar to copper and has the electricalstrength necessary for 110 kV BILL. Other materials that have suitablemechanical and electrical properties could be used for tube 123. Thetube nominally fills the annular space in the transition region andextends to internal connector 218 on the ambient temperature end andinto the splice on the cold end. In the ambient temperature region tube123 has an internal o-ring groove at 115 to inhibit coolant flow throughthis space. The internal surface tube 123 is wound on copper foil overits full length and the external surface is painted with electricallyconductive paint over that part of its length that contacts the coppershield conductor pipe, thus the internal and external surfaces of thetube conduct electricity. The purpose of the electrically conductivecoating is to eliminate partial discharge in the very small annular gapat the copper to electrical insulator interface. A small metal shim isplaced between the copper pipes and the conductive coating in theambient temperature region to fix the joined surfaces at the samepotential, thus eliminating any possibility of discharge. Advantageouslythe distance between the end of shield pipe 123 and internal phaseconnector 115 is greater then about 7 inches and preferably in excess of8 inches. The edge of shield pipe 114 is sealed to insulating tube 123using a commercial polymeric stress relief kit 29. Elements of stressrelief kit 29 are carefully applied to form a gas tight seal betweenshield pipe 114 and insulating tube 123. Therefore, stress relief kit 29has three functions: electrical stress relief, gas flow restriction, andmechanical securing of adjacent elements. Thus the set of conductingpipes (112 and 114) that passes through the transition zone of thetermination along with nested insulating tube 123 and plug 111, withinternal connectors 118 and 218 in place with stress material 9, forms arigid assembly that is free to move back and forth as a unit in theinsulated conduit.

[0043] The transition duct is sized to accept the previously describedset of concentric normal conducting pipes and insulating tubes. Theinsulated conduit 109 in FIGS. 3 and 4 preferably is stainless steel andis grounded in service. Therefore, in order to mitigate the possibilitythat any internal components would short to conduit 109 in an accidentcondition, and to provide additional thermal insulation, a G-10 sleeve122 is installed between the duct wall and the shield conductor pipe112. Sleeve 122 is of sufficient thickness to meet the BIL rating of thecable. Sleeve 122 is further sized internally to have a free-running fiton shield conductor pipe 112 and externally to allow introduction ofsealant compounds. Advantageously the space between G-10 sleeve 122 andinsulated conduit 109 is completely filled from end to end with a lowtemperature addition cured silicone elastomer compound such as DowCorning 3-6121 Encapsolating Elastomer. The elastomer compound preventscoolant flow in the annular space, thus preventing any undesiredconvective cooling in this region. The ambient temperature end of sleeve122 is sealed against gas flow on the shield conductor pipe preferablywith a spring-loaded polytetrafluoroethylene (PTFE) reciprocating seal104, such as Bal Seal 317 MB-409. A centering ring 105 is installed nextto seal 104 to take up any radial mechanical force. The centering ringis sized to have a free-running fit on shield conductor 112 and is madeof a material that has similar thermal-mechanical properties to copperand also has a low coefficient of friction against copper;advantageously a mica-filled PTFE such as Polymer Corporation Fluorosint500 may be used. Thus, the transition duct is sized so that theinsulators and conductors completely fill the duct and sealantcompounds, elastomer seals, and mechanical seals close the gaps betweenthe insulators, conductors, and transition duct.

[0044] Plug 111 in FIGS. 3 and 4 closes the inside of phase conductor104. Preferably plug 111 is fabricated of filament wound glassimpregnated with epoxy having properties similar to G-10, the center ofwhich is completely filled from end to end with a low temperatureaddition cured silicone elastomer compound 130, such as Dow Corning3-6121 Encapsolating Elastomer. Plug 111 may be fabricated entirely of asingle material that has relative low thermal conductivity and hasthermal expansion properties similar to copper. Plug 111 does not haveto be an electrical insulator and can have different embodimentsdepending on whether an external or internal capillary is used topressurize the ambient temperature housing. If an external capillary isused plug 111 is advantageously sealed an elastomeric o-ring 116 to forma hermetic seal that impedes gas flow through the annulus between plug111 and phase conductor pipe 112. If internal pressurization is employedthe annulus between plug 111 and phase conductor pipe 112 becomes thecapillary passage and is not sealed. Instead, plug 111 advantageously ismade to form a loose-running fit with phase conductor pipe 112 thatallows ample gas flow for pressurization and a double thread is cut inplug 111, one thread of which is filled with packing 129. The doublethreads advantageously are 0.5 inch pitch starting 180 degrees apart andare of a depth appropriate of packing (ie. 0.125 inch diameter and 0.1inch deep designed to receive 0.125 inch diameter GoreTex packing in thepreferred embodiment) and a length of 4.5 inches. Both liquid andgaseous coolant are free to flow in one groove and packing 129 preventsflow in the other groove so that a series of equilibrium cells areformed with the state of the fluid in each cell being determined by thetemperature of copper phase conductor pipe 112 and the pressure of thecoolant. Plug 111 has a further function in that it is sized on the coldend to control forced convection heat transfer of coolant fluid alongthe inner surface of phase conductor pipe 112.

[0045]FIG. 5(a) shows the termination to cable joint packing consistingof a grease impregnated fiberglass sleeving and FIG. 5(b) shows analternate joint packing of a fiberglass sleeve filled with grease.

[0046] The splice section of the termination, depicted in FIG. 6,provides both electrical connection between HTS and normal conductingelements and cooling for the conductors. The HTS cable is wound on aformer 202 and has as its basic elements the HTS phase conductor 203, anelectrical insulation package 205, and a coaxial HTS shield 206. Thecable end has two copper elements 217 and 222 that are threaded togetherand to former 202. The HTS phase conductors are soldered to copper end222. Copper end 222 is threaded to copper phase conductor pipe 104 andlocked in place with a jam nut 223. In this embodiment said HTS shield206 is advantageously covered with a brass sleeve 207 that is clampedonto the cable with bolted copper clamshell connector 103. In otherembodiments, said sleeve 207 may be copper and said connection betweenHTS shield 206 and sleeve 207 may be soldered. Connector 103 is furtherbolted to said copper shield conductor pipe 114 using a flange that issilver brazed to shield conductor pipe 114. The annular space betweencopper phase pipe 104 and copper shield pipe 114 is filled with anelectrical insulation package that is layered in such a way as toprovide electrical insulation, electrical stress relief, and a flow pathfor coolant. Advantageously copper phase pipe 104 has a ring of holesfor coolant flow located at 215 and copper shield pipe 114 has a ring ofholes for coolant flow located at 229. The flow path for coolant throughthe annulus is established by a layer of thin-wall Teflon tubes 216 thatare twisted in a helical pattern as the diameter changes such that theyform a single dense layer of tubing having no gaps between tubes. Theoutside of HTS phase conductor 203, its solder joint to copper cable end222, and end 222 are covered by a single layer of semiconductor tapematerial. Layers of Cryoflex™ insulation 213 and 219 are then wound overthe semiconductor tape to form a cone for Teflon® tubes 216 to lay onand to form an electrical stress relief cone at 210 that extends fromHTS shield 206 to copper shield conductor pipe 209. Electrical stressrelief cone 210 is formed by a single complete layer of semiconductortape material held in place by a single complete layer of copper tapehaving conducting adhesive and over wound with layers of Cryoflexinsulation 219 out to the inside diameter of shield conductor pipe 114.Semiconductor tape material from stress cone 107 is extended over theoutside surface of Cryoflex insulation layers 213 to Teflon tubes 216 toeliminate electrical stress at holes 215. The outside of Teflon tubes216 and the tapered section of G-10 sleeve 123 is over wound with layersof Cryoflex insulation to the inside diameter of shield conductor pipe114. The various layers of electrical insulation and semiconductor tapeare fastened in place, when required, by Kapton tape.

[0047]FIG. 6 depicts the feed termination. In this termination coolantenters at 232, flows along the outside of copper shield conductor pipe114 through annular heat transfer gap 227, through holes 215, throughTeflon tubes 216, along the outside of copper phase conductor pipe 104through annular heat transfer gap 225, through holes 229, and then alongthe inside of copper phase conductor pipe 104 through heat transfer gap224 to the inside of the cable. Flow through the splice in the returnend is in the opposite direction where coolant flows to the outside ofthe cable and then back to the feed end where it exits through 230. Heattransfer gaps 227, 224, and 225 are sized to maintain the cold sectionof the termination at the desired operating temperature.

[0048] The outside of heat transfer gap 227 envelope in FIG. 6 is formedby G-10 sleeve 107. Sleeve 107 has a stainless steel flange at one endthat is fastened to insulated conduit 109 using bolts. Sleeve 107differs between the feed and return ends. FIG. 6 depicts the feed endthat has a spring-loaded PTFE face seal 127 between G-10 sleeve 107flange and insulated conduit 109. The return end requires no such seal.The stainless steel flange on G-10 sleeve 107 for the return end,however, has several holes that allow free passage of coolant across theflange from the inside to the outside of the flange. G-10 sleeve 107 hasa spring loaded sliding seal 212 that rides against copper shieldconductor pipe 114. Sliding seal 212, depicted in FIG. 7, uses 0.062inch thick GoreTex gasket material 304 to form a seal between G-10sleeve 107 and copper shield conductor 114. The GoreTex gasket is heldin place with cover plate 303 that has a record groove surface to holdthe GoreTex gasket in place. GoreTex gasket 304 is energized by ahelical spring 305 such as Bal Seal 107LBA-(2.500)-50W-2 spring fittedinside retaining ring 301. Centering ring 105 functions as a sleevebearing supporting copper shield conductor 114.

[0049] Although this invention describes a connector for a single phasesuperconducting cable, it will be understood by one skilled in the artthe above invention is also useable for multi-phase cables.

[0050] Although specific embodiments have been illustrated anddescribed, it will be obvious to those skilled in the art that variousmodifications may be made without departing from the essence of theinvention.

What is claimed is:
 1. An isobaric superconducting cable terminationconsisting of: a set of normal conducting connectors, one for eachsuperconducting conductor to be joined electrically to an externalambient temperature cable, each termination conductor having a cold endwhich connects to a respective superconducting conductor and a warm endwhich connects to a hermetic ambient temperature pressurized bushing,each normal termination conductor being separated from the other byelectrical insulators, said termination conductors and insulatorscontained in a thermally insulated termination conduit, consists ofthree parts: a) a cold housing for making connections between saidsuperconductors and said normal termination conductors; b) a warmhousing for making connections between the normal termination conductorsand the internal connection of the ambient pressurized bushing; and c) atransition duct connecting said cold housing with said warm housingthrough which said normal termination conductors and insulators pass,said transition duct being sized so that said insulators and conductorscompletely fill the duct, said insulators and conductors being sealedusing sealant compounds, elastomer seals, and mechanical seals so as toprevent liquid coolant from flowing from the cold housing to the warmhousing, one or more capillary passages through or parallel to thetransition duct connecting said cold housing to said ambient housing toallow gas to flow to maintain pressure equilibrium between said coldhousing and said warm housing, said normal termination conductors andsaid transition duct being of size and length so as to minimize the heatflow through said normal conductors from said ambient temperaturehousing to said cold housing.
 2. The apparatus of claim 1 in which saidnormal conductors are capable of sliding within said transition duct soas to allow thermal expansion and contraction of said superconductingcable.
 3. The apparatus of claim 1 in which said thermal insulation isprovided by a vacuum insulated space surrounding said cold housing andsaid transition duct.
 4. The apparatus of claim 1 in which said normaltermination conductors consist of rigid coaxial pipes.
 5. The apparatusof claim 1 in which the connection between said superconducting cableconduit and said termination conduit is provided by a bayonet couplingsealed by grease packed fiberglass sleeving.
 6. The apparatus of claim 1further consisting of a coaxial splice connecting said coaxial insulatedsuperconductors and said coaxial insulated normal conductors, saidinsulation having a conical passage provided by a set of tubing betweensaid insulators which allows liquid coolant to flow through theinsulation layer.
 7. A termination for connecting high temperaturesuperconducting (HTS) cable located within a cable carrier to warmexternal bushings, said HTS cable having inner and outer HTS conductorsinsulated by a dielectric tape insulator, said termination comprising: avacuum house weldment having a first and second end; an inner housinglocated within said vacuum house weldment, said inner housing connectedto said cable carrier and enclosing an end of said HTS cable;termination piping sealably connected to said second end of said vacuumhouse weldment, said piping containing said external bushings; an innerresistive conductor extending axially at least partially through saidvacuum house weldment, said inner housing and said termination piping,said inner resistive conductor having a cold end and a warm end; anouter resistive conductor, coaxial to said inner resistive conductor andseparated therefrom by a first insulation sleeve, said outer resistiveconductor having a cold end and a warm end; connecting means forconnecting said inner resistive conductor to said inner HTS conductorand for connecting said outer resistive conductor to said outer HTSconductor; cooling means for supplying coolant to the cold end of saidinner and outer conductor; a first bushing connector located within saidtermination piping for connecting said inner resistive conductor to afirst bushing; and a second bushing connector located within said innerhousing for connecting said outer resistive conductor to a secondbushing.
 8. The termination of claim 7 further comprising a flow valveconnected to said termination piping for allowing flow of said coolantinto said termination piping.
 9. The termination of claim 7 wherein saidtemperature of said termination piping is above the flash temperature ofsaid coolant.
 10. The termination of claim 7 wherein said terminationpiping is connected to said vacuum house weldment by a connection hose.11. The termination of claim 7 wherein said pressure in said terminationpiping is equal to said pressure in said vacuum house weldment innerhousing.
 12. The termination of claim 7 further includingsuperinsulation between said inner housing and said first insulationsleeve.
 13. The termination of claim 7 further including a slidable sealallowing expansion and contraction of said inner conductor.
 14. Thetermination of claim 7 wherein said coolant is liquid nitrogen.
 15. Thetermination of claim 7 wherein said termination piping is made offiberglass-epoxy.
 16. A termination for connecting high temperaturesuperconducting (HTS) cable located within a cable carrier to warmexternal bushings, said HTS cable having inner and outer HTS conductorsinsulated by a dielectric tape insulator, said termination comprising: avacuum house weldment having a first and second end; an inner housinglocated within said vacuum house weldment said inner housing connectedto said cable carrier and enclosing an end of said HTS cable;termination piping sealably connected to said second end of said vacuumhouse weldment, said piping containing said external bushings; an innerresistive conductor extending axially at least partially through saidvacuum house weldment, said inner housing and said termination piping,said inner resistive conductor having a cold end and a warm end; anouter resistive conductor, coaxial to said inner resistive conductor andseparated therefrom by a first insulation sleeve, said outer resistiveconductor having a cold end and a warm end; tubing extending betweenlayers of said dielectric tape, said tubing for carrying cryogeniccooling fluid between said outer resistive conductor and said innerresistive conductor; at least one heat transfer region fluidly connectedto said tubing, said at least one heat transfer region for cooling saidinner and outer resistive conductors; at least one flow pipe extendingthrough said housing to said vacuum house weldment inner housing, saidflow pipe for providing flow of said cryogenic cooling fluid within andwithout said inner housing; a first bushing connector located withinsaid termination piping for connecting said inner resistive conductor toa first bushing; a second bushing connector located within said innerhousing for connecting said outer resistive conductor to a secondbushing; a first connection means for connecting said inner resistiveconductor to said inner HTS conductor; a second connection means forconnecting said outer resistive conductor to said outer HTS conductor;and a coolant source in communication with said inner housing forsupplying cryogenic cooling fluid.
 17. The termination of claim 16further comprising a first seal located at the engagement of said atleast one flow pipe and said inner housing, said seal for limiting theflow of said cryogenic cooling fluid.
 18. The termination of claim 16wherein said cooling fluid is liquid nitrogen.
 19. The termination ofclaim 16 further comprising a seal located at the connection of saidpiping and said housing for separating said weldment from said piping.20. The termination of claim 16 wherein said seal is a sliding seal. 21.The termination of claim 16 further comprising conducting andsemiconducting tape wound between and around said HTS inner conductor,said inner resistive conductor, and said outer resistive conductorforming at least one stress cone.
 22. The termination of claim 16wherein said tubing is made of Teflon.
 23. The termination of claim 16wherein said at least one heat transfer region comprises: a first zonedefined by an annular space between said first insulation sleeve andsaid outer resistive conductor.
 24. The termination of claim 16 whereinsaid at least one heat transfer region includes: a second zone definedby an annular space between said first insulation sleeve and said innerresistive conductor.
 25. The termination of claim 16 wherein said atleast one transfer region includes: a third zone defined by an annularspace between a plug located at the interior of said inner resistiveconductor and said inner resistive conductor.
 26. The termination ofclaim 16 wherein said at least one heat transfer region comprises: afirst zone defined by an annular space between said first insulationsleeve and said outer resistive conductor; a second zone defined by anannular space between said first insulation sleeve and said innerresistive conductor; and a third zone defined by an annular spacebetween a plug located at the interior of said inner resistive conductorand said inner resistive conductor.
 27. The termination of claim 16further including a flow valve connected to said termination piping forallowing flow of said cryogenic cooling fluid into said terminationpiping.
 28. The termination of claim 16 wherein said temperature of saidtermination piping is above the flash temperature of said cryogeniccooling fluid.
 29. The termination of claim 16 wherein said terminationpiping is connected to said vacuum house weldment by a connection hose.30. The termination of claim 16 wherein said pressure in saidtermination piping is equal to said pressure in said inner housing. 31.The termination of claim 16 further including superinsulation betweensaid inner housing and said first insulation sleeve.
 32. The terminationof claim 16 further including a slidable seal allowing expansion andcontraction of said inner conductor.
 33. The termination of claim 16further including a centering ring for maintaining the area of at leastone heat transfer zone.
 34. The termination of claim 16 wherein saidtermination piping is made of fiberglass-epoxy.
 35. The termination ofclaim 16 wherein said bushing connections comprise copper braidedstraps.
 36. The termination of claim 16 wherein said inner and outerconductors are copper.